Thermoplastic-photoconductor holographic recording medium has generally been in the form of several transparent layers over a transparent substrate. Thus a substrate such as nesa glass or a flexible substrate such as Mylar is first coated thereon an optically transparent electrically conductive layer, then a photoconductor layer, and finally a thermoplastic layer. Previous thermoplastic holographic recording media suffer the problem of decaying charge contrast at the interface of the photoconductive and the thermoplastic layer during hologram develpment, resulting in low diffraction efficiency or in worse cases no deformation at all.
The charge contrast created by the holographic light pattern through the action of the photoconductor resides at the interface of the thermoplastic layer and the photoconductive layer. During thermal development the charge contrast tends to diminish due to increased electrical conductivity of the thermoplastic layer. As a result, the driving force for the surface deformation is weakened, thereby the deformation is much less than it could be had the charge contrast maintained intact. There exists a large electrical potential difference between the bright fringe and the dark fringe at the interface and this potential difference has a tendency to diminish the charge contrast. At temperatures below the softening temperature of the thermoplastic, however, the electrical conductivity is not high enough to wipe out the charge contrast. For example, with a polyvinyl carbazole (PVK) layer as the photoconductive layer, the electron mobility is sufficiently low at temperatures below 100.degree. C to prevent charge smearing, but with the thermoplastic layer having a softening temperature of say 50.degree. C, the charge contrast can be wiped out when the softening temperature is reached for a sufficiently long time.
In this invention we introduce a new layer between the thermoplastic layer and the photoconductive layer with the following properties: (a) It is non-photoconductive at the wavelength of the holographic recording but is photoconductive at a different wavelength. (b) It is electrically insulating at temperatures used for thermal development, that is, extremely low surface conductivity at the development temperature. The charge contrast created by the holographic exposure will, therefore, reside at the interface of this new layer and the photoconductive layer and during thermal development the charge contrast will remain intact. We can thus have a large driving force for deformation and achieve controllable and appreciable deformation.
After the thermal development, charges will reside across this new layer and they can be removed by using a wavelength to which this new layer is sensitive. As an example of fabricating such a device, a trinitrofluorenone (TNF) doped PVK layer is first coated on, say, indium oxide on glass, and then a special solvent such as xylene is used to leach out an appropriate amount of TNF from the surface of the doped PVK. This procedure generates the new layer mentioned in this invention because pure PVK (PVK stripped TNF) is photoconductive only at ultraviolet (UV) wavelengths and pure PVK is also electrically insulating at temperatures up to 180.degree. C. Advantages of our present invention are: (a) the hologram diffraction efficiency (.eta.) can be higher, (b) the dynamic range of .eta. vs. heat energy is greatly increased while in prior art .eta. is critically dependent on heat energy, (c) thermoplastic with relatively high electrical conductivity can be used.